2_D Modeling of Sabalan Magnetotelluric Data using Nonlinear Conjugate Gradient and Smoothness_Constrained Least Square Methods

Summary The main objective of this research work is to investigate the subsurface structures in Sabalan geothermal field using two-dimensional (2-D) nonlinear conjugate gradient and smoothness-constrained least square inversion methods of magnetotelluric (MT) data. To achieve the goal, 8 MT sites in the Sabalan area have been selected along one profile. Dimensionality analysis using the WALDIM code and impedance polar diagrams indicated that the subsurface structures of the selected sites are One-dimensional (1-D) or 2-D at shallow depths, whereas they are mainly three-dimensional (3-D) at middle and lower depths. Tipper and induction arrows have been used to determine the geoelectrical strike. As a result, NE-SW direction has been identified as the major strike direction of subsurface structures in the area. Spatial averaging method has also been used to remove the galvanic distortions. After setting up the modeling parameters and appropriate mesh design, 2-D inversion of the MT data has been performed using nonlinear conjugate gradient and smoothness-constrained least square methods. The obtained 2-D models have illustrated very well the geothermal structures including cap rock, reservoir and the heat source. Moreover, there is a close correspondence between these models and past investigations in the area.
Introduction The Sabalan geothermal field is located on the western slopes of Sabalan mountain in the Moil valley in Ardabil province. Warm and hot springs w are found within the Moil valley in the area. The four major units, originally identified and used for the original geologic study and mapping at northwest of Sabalan, are in order of increasing age: Quaternary alluvium, fan and terrace deposits; Pleistocene post-caldera tracheyandesite flows, domes and lahars; Pleistocene syn-caldera tracheydacitic to tracheyandesitic domes, flows and lahars; Pleistocene pre-caldera tracheyandesitic lavas, tuffs and pyroclastics. Fractures and faults can play an important role in geothermal fields, as fluid mostly flows through them in the source rocks. Structural study confirms two major types of structural setting: a set of linear faults and several ring-faults. Sabalan area has been under studies including geology, hydrology, geochemistry and resistivity survey since 1998. Three deep exploratory wells were consequently drilled from 2002 to 2004 within the moil valley. Measured temperature at wells drilled in the area varies between 160 to 250ºC. In 2006, a hydrological model of the region has been proposed. In 2007 and 2009, considering the proposed hydrological model, MT survey was conducted to determine the possible center of the geothermal system and identify the drilling targets. In this research work, the subsurface structures in Sabalan geothermal field is investigated using 2-D nonlinear conjugate gradient and smoothness-constrained least square inversion methods of the acquired MT data from 8 MT sites in the area. Based on the inversion results, the geoelectrical model or specifications of subsurface structures in the area will be estimated.
Methodology and Approaches To investigate the subsurface structures in the Sabalan area using 2-D nonlinear conjugate gradient and smoothnessconstrained least square inversion methods of MT data, 8 MT sites were selected from 78 MT sites measured in 2007 and 2009 MT survey, and then, the MT data of these 8 sites were used in the inversion process. The survey utilized a Phoenix MTU-5A data acquisition system, which is a standard five-channel system recording both two electric and three magnetic field components. The raw time series data were processed using SSMT2000 software. The processed data were then used to obtain the impedance tensors from which the apparent resistivity and phase data in a frequency range of 0.0011-320 Hz were calculated. WALDIM code and Polar diagrams were used for dimensionality analysis of MT data. Static shift correction based on the similarity between the MT curves of adjacent sites was utilized to remove the effects of galvanic distortions. After dimensionality analysis and static shift correction, and then, setting up the modeling parameters and appropriate mesh design, we performed 2-D inversion of the MT data by employing nonlinear conjugate gradient and smoothness-constrained least square methods in MT2InvMatlab code and WinGLink software. Because the fitting of the transverse magnetic (TM) mode data in the 2-D inversion can be more easily achieved even for a 3-D structure, often the TM mode data is only used for 2-D inversion of MT data in geothermal fields. Therefore, we have only shown the inversion results of TM mode data and interpretation has also made for the obtained model of the TM mode data.
Results and Conclusions As a result of the inversion of the MT data, 2-D models of subsurface geoelectrical structures have been obtained, and consequently, the cap rock, reservoir and heat source of Sabalan geothermal field have been identified. Moreover, the ACB algorithm introduced by Yi et al. (2003) has been used to stabilize the inversion process and provide a highresolution optimal earth model in the MT2InvMatlab code. Constructing the forward and inversion mesh in the MT2InvMatlab code is also made automatically.